This invention is generally in the field of implantable miniaturized devices that provide controlled delivery or exposure of molecules or smaller devices. More specifically, the invention relates to implantable microchip devices.
When an implanted medical device such as a pacemaker is placed in the body, one needs to consider both the impact of the body on the device and the impact of the device on the body. The environment is electrically conductive and relatively corrosive, which can compromise the integrity and performance of the device electrically or mechanically. The presence of a foreign object in the body triggers the body's defense mechanisms, which can impair the functioning of the body and/or the device. One commonly used method to enhance the body's acceptance of the implanted device is coat the device with a biocompatible coating material.
Pinhole-free, conformal coatings are designed to seal devices from liquids and gases, while protecting and electrically insulating the device. Such coatings are typically used to conform to the surface contours of an assembled printed circuit board and electronic components. They protect the circuitry from the environment, prevent damage, provide mechanical strength, and increase dielectric strength between components. Examples of conformal coatings include silicone, urethane, acrylic, and epoxy. Typical methods for depositing these coatings include dipping, spraying, spin coating, and ultraviolet (UV) curing. U.S. Pat. No. 5,510,138 discloses a method of applying such coatings using hot melt dispensing equipment. Although these conformal coatings and processes are adequate for coating electronics and circuits, they may be inappropriate for microchip chemical delivery devices, such as described in U.S. Pat. Nos. 5,797,898 and 6,123,861 to Santini, Jr. et al. and in Nature, 397:335–38 (1999) and Angewandte Chemie, 39:2396–407 (2000). Most of the typical coatings require a high temperature or UV cure. Some are also solvent-based coatings, which could adversely react with the reservoir contents (e.g., drug molecules or a device) in the microchip device. These types of coatings also may be unacceptable due to trapped air bubbles and uneven coating.
Another coating material is parylene, which is used in numerous medical applications. Parylene is the common name of a family of vapor-deposited conformal coatings based on para-xylylene and its derivatives. U.S. Pat. Nos. 5,393,533 and 5,288,504 disclose the use of parylene in controlled release applications involving the encapsulation of drugs and cells for therapeutic applications. Catheters and other molded surgical devices can be parylene coated to protect the device against the corrosive effects of biofluids and can also aid in the release of these devices from the fabrication molds. U.S. Pat. No. 5,425,710 discloses using parylene coating to coat a sleeve of a dilation catheter balloon to protect it during insertion. As disclosed in U.S. Pat. No. 5,824,049, stents and prostheses can be parylene coated to protect them and allow cells to proliferate on them. Parylene provides corrosion resistance and electrical insulation on sensors implanted in the body without altering the device operation. U.S. Pat. No. 5,067,491 describes a blood pressure monitoring device coated in parylene to protect the sensor from the effects of the blood, ions, and water. Both the lumen and the outside of needles and probes can be coated with parylene to create a smooth surface. These needles may be used to make microelectrodes, as disclosed in U.S. Pat. No. 5,524,338. Implantable pacemakers and defibrillators can be sealed with parylene to protect and electrically insulate the devices.
One difficulty in using parylene to coat a microchip device would be that the reservoir caps over each reservoir of the completed microchip device must not be coated, in order for the device to operate. Therefore, the coating process would need to be followed by a selective removal process. Other types of implantable devices have a similar need to remove parylene from a portion of the device. For example, U.S. Pat. No. 5,925,069 (Sulzer Intermedics) discloses using a pulsed excimer laser to remove parylene coating from the surface of an implantable cardiac pulse generator to expose a defined region of the case to serve as an electrode. A UV-resistant mask or stencil between the device and the laser beam is used to create windows or openings in the parylene coating. This patent also discloses the use of plasma etching to remove parylene in patterns having defined shapes. In this process, the organic parylene reacts with the ionized oxygen plasma to form carbon dioxide gas and water vapor, which are removed by vacuum. A mask can create patterns of various shapes in the parylene by protecting certain areas from etching.
U.S. Pat. No. 5,562,715 discloses a silicone rubber or parylene coated pacemaker with detachable tabs that remove a portion of the coating and expose the electrodes. Windows in the parylene coating are patterned using a process that includes masking select surface areas of the device with tape, coating the entire surface of the device (masked and unmasked) with parylene, and then removing the tape to expose the select surface areas.
U.S. Pat. No. 4,734,300 discloses a process for the selective removal of parylene by contacting the areas of parylene to be removed with a chemical substance, such as tetrahydrofuran, to loosen the parylene coating so that it can be physically removed. A knife is used to score the parylene coating.
Such masking techniques may not be readily adaptable for masking individual tiny reservoir caps, which may, for example, be positioned in a closely packed array in a microchip device. For example, it may be difficult to create well-defined boundaries between coated and uncoated areas in very small microchip devices.
In one process of assembling microchip chemical delivery devices, it is necessary to seal the reservoir openings (distal the reservoir caps) after filling the reservoirs with the drug molecules or other reservoir contents. It would be advantageous to be able to seal the reservoir openings with a material that is compatible with the reservoir contents.
It would be desirable to provide microchip devices having a coating which enhances biocompatibility of the device and protects and insulates the device electronics, and which is compatible with the reservoir contents. It also would be desirable to provide a method of conformally coating a microchip device to seal the drug reservoirs, to electrically insulate the electrical connections, and to provide a biocompatible outer surface. It also would be desirable to provide microfabrication techniques for use in patterning a conformal coating on a microchip device so as to selectively pattern well-defined microscopic openings in the coating which correspond to the reservoir caps of the microchip device.